Austenitic heat resistant alloy, heat resistant pressure member comprising the alloy, and method for manufacturing the same member

ABSTRACT

An austenitic heat resistant alloy, which comprises by mass percent, C: over 0.02 to 0.15%, Si≦2%, Mn≦3%, P≦0.03%, S≦0.01%, Cr: 28 to 38%, Ni: over 40 to 60%, Co≦20% (including 0%), W over 3 to 15%, Ti: 0.05 to 1.0%, Zr: 0.005 to 0.2%, Al: 0.01 to 0.3%, N≦0.02%, and Mo&lt;0.5%, with the balance being Fe and impurities, in which the following formulas (1) to (3) are satisfied has high creep rupture strength and high toughness after a long period of use at a high temperature, and further it is excellent in hot workability. This austenitic heat resistant alloy may contain a specific amount of one or more elements selected from Nb, V, Hf, B, Mg, Ca, Y, La, Ce, Nd, Sc, Ta, Re, Ir, Pd, Pt and Ag. P≦3/{200(Ti+8.5×Zr)} . . . (1), 1.35×Cr≦Ni+Co≦1.85×Cr . . . (2), Al≧1.5×Zr . . . (3).

This application is a continuation of the international applicationPCT/JP2009/060837 field on Jun. 15, 2009, the entire content of which isherein incorporated by reference.

TECHNICAL FIELD

The present invention relates to an austenitic heat resistant alloy,which has a high temperature strength far higher than that of aconventional heat resistant alloy, and is excellent in toughness after along period of use, and also excellent in hot workability, and relatesto a heat resistant pressure member comprising the said alloy, and alsoa method for manufacturing the same member. More particularly, thepresent invention relates to an austenitic heat resistant alloy whichcontains 28 to 38 mass % of Cr, which is excellent in high temperaturestrength, especially creep rupture strength, and is excellent intoughness after a long period of use due to high structural stability.Further it has remarkably improved hot workability, especially hightemperature ductility at 1150° C. or higher, being used as a pipematerial, a plate material for a heat resistant pressure member, a barmaterial, forgings, and the like for a boiler for power generation, aplant for chemical industry, and the like, and relates to a heatresistant pressure member comprising the said alloy, and a method formanufacturing the same member.

BACKGROUND ART

Conventionally, for a boiler used in a high temperature environment, achemical plant, and the like, a so-called “18-8 type austeniticstainless steel” such as SUS 304H, SUS 316H, SUS 321H, SUS 347H, and thelike has been used as an equipment material.

However, in recent years, the conditions under which this equipment wasused in a high temperature environment have become extremely severe, andtherefore the performance requirements of material to be used havebecome stringent; under these circumstances, the above-described 18-8type austenitic stainless steel, having been used conventionally, hasbecome remarkably insufficient in high temperature strength, especiallycreep rupture strength. Therefore, in order to solve the said problem,an austenitic stainless steel, with improved creep rupture strength, hasbeen developed by containing proper amounts of various elements.

On the other hand, nowadays, in the field of a boiler for thermal powergeneration, for example, a project is underway to raise steamtemperature, which has conventionally been about 600° C. at the most, to700° C. or higher. In this case, the temperature of a member to be usedfar exceeds 700° C., and therefore, even if the above-described newlydeveloped austenitic stainless steel is used, the creep rupture strengthand corrosion resistance are insufficient.

Generally, in order to improve the corrosion resistance, it is effectiveto increase the content of Cr in the steel. However, in the case wherethe Cr content is increased, for example, as seen in SUS 310S whichcontains about 25 mass % of Cr, the creep rupture strength at 600 to800° C. rather becomes lower than that of 18-8 type stainless steels,and the toughness is deteriorated due to the precipitation of u phase.Further, even if the Cr content is increased, about 25 mass % of Crcannot provide sufficient corrosion resistance in a severe corrosiveenvironment.

Thus, the Patent Documents 1 to 7 disclose heat resistant alloys inwhich the contents of Cr and Ni are increased, and moreover one or morekinds of Mo and W are contained in order to improve the creep rupturestrength as high temperature strength.

Further, in order to meet the increasingly stringent requirements forhigh temperature strength characteristics, especially the requirementsfor creep rupture strength, the Patent Document 8 discloses a heatresistant alloy which contains, by mass %, 28 to 38% of Cr and 30 to 50%of Ni, and the Patent Documents 9 to 14 disclose heat resistant alloyswhich contain, by mass %, 28 to 38% of Cr and 35 to 60% of Ni. For allof the heat resistant alloys proposed in the Patent Documents 8 to 14,the creep rupture strength is further improved by utilizing theprecipitation of α-Cr phase of a body-centered cubic structureconsisting mainly of Cr.

CITATION LIST Patent Document

-   Patent Document 1: JP 60-100640 A-   Patent Document 2: JP 61-174350 A-   Patent Document 3: JP 61-276948 A-   Patent Document 4: JP 62-63654 A-   Patent Document 5: JP 64-55352 A-   Patent Document 6: JP 2-200756 A-   Patent Document 7: JP 3-264641 A-   Patent Document 8: JP 7-34166 A-   Patent Document 9: JP 7-70681 A-   Patent Document 10: JP 7-216511 A-   Patent Document 11: JP 7-331390 A-   Patent Document 12: JP 8-127848 A-   Patent Document 13: JP 8-218140 A-   Patent Document 14: JP 10-96038 A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The heat resistant alloys disclosed in the Patent Documents 1 to 7cannot necessarily obtain sufficiently high creep rupture strength in asevere environment in which the steam temperature is 700° C. or higher.

Also, it cannot be said that the heat resistant alloys disclosed in thePatent Documents 8 to 14 are sufficient in creep rupture strength thathas been required to be high in recent years. Further, the heatresistant alloys disclosed in the Patent Documents 8 to 14 are sometimesinsufficient in toughness after a long period of use depending on thealloy composition thereof. Moreover, regarding these heat resistantalloys, it has been desired to further improve the hot workability,especially the hot workability on the high temperature side of 1150° C.or higher. The reason for this is that in a case where a seamless steelpipe is manufactured by using a material having a poor hot workability,the seamless steel pipe is often manufactured by the hot extrusionprocess, and if the hot workability on the high temperature side of1150° C. or higher is insufficient, the internal temperature of thematerial becomes higher than the heating temperature due to a work heatgeneration, so that defects, such as two-piece cracks and scabs, areformed. If the hot workability on the high temperature side of 1150° C.or higher is insufficient, in a piercing process using a piercing millof, for example, a Mannesmann-mandrel mill system, the above-describeddefects are formed in the same way.

In view of the above-mentioned state of affairs, the objective of thepresent invention is to provide an austenitic heat resistant alloycontaining 28 to 38 mass % of Cr, which has high temperature strength,especially creep rupture strength, which is far higher than that of theconventional heat resistant alloys, especially the heat resistant alloysdisclosed in the Patent Documents 8 to 14. It has high toughness becausethe structural stability is excellent even after a long period of use ata high temperature, and further it has remarkably improved hotworkability, especially high temperature ductility at 1150° C. orhigher.

Means for Solving the Problems

The present inventors examined the creep rupture strength, structuralstability in a long period of use, hot workability, and the like byusing various heat resistant alloys containing, by mass %, 28 to 38% ofCr and more than 40% to not more than 60% of Ni as base components andcapable of utilizing precipitation strengthening of the α-Cr phase. As aresult, the present inventors obtained the following findings (a) to(g).

(a) If a proper amount of W is contained, an Fe₂W type Laves phaseand/or an Fe₇W₆ type μ phase precipitate, and therefore the creeprupture strength is significantly improved.

(b) In the case where 28 to 38% of Cr is contained, and if W can bedissolved into the precipitated α-Cr phase, the growing and coarseningof the α-Cr phase during a long period of use at a high temperature arerestrained, a sudden decrease in creep rupture strength on the long timeside does not occur.

(c) Conventionally, it has generally been thought that Mo and W haveequivalent operational advantages; however, in the case where Mo iscompositely contained in an alloy containing W and 28 to 38% of Cr, theσ phase sometimes precipitates on the long time side. Therefore, thecreep rupture strength, ductility, and toughness may decrease.

(d) By properly controlling the content of Ni, which is an austenitestabilizing element, with respect to the Cr content, the precipitationof the σ phase during a long period of use at a high temperature can berestrained stably and reliably, and moreover, the optimum amount of α-Crphase can be precipitated. In the case where the alloy compositelycontains Co, by controlling the contents of Ni and Co with respect tothe Cr content so that the sum of the contents of Ni and Co (that is,“Ni+Co”) is proper, the precipitation of the σ phase, during a longperiod of use at a high temperature, can be restrained stably andreliably, and moreover, the optimum amount of α-Cr phase can beprecipitated.

(e) Zr, which has generally been known as a “grain boundarystrengthening element”, is competent for improving the creep rupturestrength in the case of the heat resistant alloy capable of utilizingthe precipitation strengthening of α-Cr phase. Further, by properlycontrolling the content of Al in accordance with the content of Zr, thecreep rupture strength is significantly improved.

(f) Ti also improves the creep rupture strength of the heat resistantalloy capable of utilizing the precipitation strengthening of α-Crphase. By containing Ti compositely with Zr, the precipitation of α-Crphase is further promoted, whereby the creep rupture strength can befurther enhanced.

(g) Since Ti and Zr lower the melting point of the heat resistant alloy,the hot workability, especially the hot workability on the hightemperature side of 1150° C. or higher, decreases, and further the hightemperature crack resistance at the time of welding may decrease.However, by properly controlling the content of P in accordance with thecontents of Ti and Zr, the hot workability on the high temperature sideof 1150° C. or higher can be improved stably and reliably while a highcreep rupture strength is maintained. Further, the high temperaturecrack resistance at the time of welding can be improved.

The present invention has been accomplished on the basis of theabove-described findings. The main points of the present invention areaustenitic heat resistant alloys shown in the following (1) to (3), aheat resistant pressure member shown in the (4), and a method formanufacturing a heat resistant pressure member shown in the (5).

(1) An austenitic heat resistant alloy, which comprises by mass percent,C: more than 0.02% to not more than 0.15%, Si: 2% or less, Mn: 3% orless, P: 0.03% or less, S: 0.01% or less, Cr: 28 to 38%, Ni: more than40% to not more than 60%, W: more than 3% to not more than 15%, Ti: 0.05to 1.0%, Zr: 0.005 to 0.2%, Al: 0.01 to 0.3%, N: 0.02% or less, and Mo:less than 0.5%, with the balance being Fe and impurities, in which thefollowing formulas (1) to (3) are satisfied:

P≦3/{200(Ti+8.5×Zr)}  (1),

1.35×Cr≦Ni≦1.85×Cr  (2),

Al≧1.5×Zr  (3);

wherein each element symbol in the equations (1) to (3) represents thecontent by mass % of the element concerned.

(2) An austenitic heat resistant alloy, which comprises by mass percent,C: more than 0.02% to not more than 0.15%, Si: 2% or less, Mn: 3% orless, P: 0.03% or less, S: 0.01% or less, Cr: 28 to 38%, Ni: more than40% to not more than 60%, Co: 20% or less, W: more than 3% to not morethan 15%, Ti: 0.05 to 1.0%, Zr: 0.005 to 0.2%, Al: 0.01 to 0.3%, N:0.02% or less, and Mo: less than 0.5%, with the balance being Fe andimpurities, in which the following formulas (1), (3) and (4) aresatisfied:

P≦3/{200(Ti+8.5×Zr)}  (1),

Al≧1.5×Zr  (3),

1.35×Cr≦Ni+Co≦1.85×Cr  (4);

wherein each element symbol in the equations (1), (3) and (4) representsthe content by mass % of the element concerned.

(3) The austenitic heat resistant alloy according to the above (1) or(2), which further contains, by mass percent, one or more elements ofone or more groups selected from the

1

to

3

groups listed below in lieu of a part of Fe:

1

Nb: 1.0% or less, V: 1.5% or less, Hf: 1% or less and B: 0.05% or less;

2

Mg: 0.05% or less, Ca: 0.05% or less, Y: 0.5% or less, La: 0.5% or less,Ce: 0.5% or less, Nd: 0.5% or less and Sc: 0.5% or less;

3

Ta: 8% or less, Re: 8% or less, Ir: 5% or less, Pd: 5% or less, Pt: 5%or less and Ag: 5% or less.

(4) A heat resistant pressure member excellent in creep resistanceproperties and structural stability in a high temperature range, whichis made from the austenitic heat resistant alloy according to any one ofthe above (1) to (3).

(5) A method for manufacturing the heat resistant pressure memberexcellent in creep resistance and structural stability in a hightemperature range according to the above (4), wherein the austeniticheat resistant alloy according to any one of the above (1) to (3) istreated in sequence by the following steps (i), (ii) and

step (i): heating to 1050 to 1250° C. at least once before final hot orcold working;

step (ii): carrying out a final hot or cold plastic working such thatthe reduction of area is 10% or more;

step (iii): carrying out a final heat treatment in which cooling isperformed after heating and holding at a temperature in the range of1100 to 1250° C.

The term “impurities” so referred to in the phrase “the balance being Feand impurities” indicates those impurities which come from ores andscraps as raw materials, environments, and so on in the industrialproduction of alloys. Also, the “high temperature range” is atemperature range in which creep deformation occurs, and means atemperature range of 600° C. or higher in the alloy of the presentinvention, and about 600 to 900° C. considering the upper limit in termsof strength.

EFFECTS OF THE INVENTION

The austenitic heat resistant alloy according to the present invention,has high temperature strength, especially creep rupture strength, higherthan that of the conventional heat resistant alloys, and also has hightoughness because the structural stability is excellent even after along period of use at a high temperature. Further it is excellent in hotworkability, especially high temperature ductility at 1150° C. orhigher. Therefore, this austenitic heat resistant alloy can be suitablyused as a pipe material, a plate material for a heat resistant pressuremember, a bar material, forgings, and the like for a boiler for powergeneration, a plant for chemical industry and so on.

MODES FOR CARRYING OUT THE INVENTION

Hereunder, the requirements of the present invention are described indetail. In the following description, the symbol “%” for the content ofeach element means “% by mass”.

(A) Austenitic Heat Resistant Alloy

C: more than 0.02% to not more than 0.15%

C (carbon) forms carbides which have an effect of ensuring tensilestrength and creep rupture strength that are necessary when the alloy isused in a high temperature environment. In order to obtain this effect,a content of C more than 0.02% is necessary. However, even if an amountof more than 0.15% of C is contained, the amount of undissolved carbidesafter the solution heat treatment merely increases; C does notcontribute to the improvement in high temperature strength, and othermechanical properties such as toughness and the weldability aredeteriorated. Therefore, the content of C is set to more than 0.02% tonot more than 0.15%. The preferable content range of C is more than0.03% to not more than 0.13%, and the further preferable range thereofis more than 0.05% to not more than 0.12%.

Si: 2% or less

Si (silicon) is added as a deoxidizing element. Si also is an elementeffective in raising oxidation resistance, steam oxidation resistanceand so on. However, if the Si content increases and especially exceeds2%, the formation of intermetallic compounds such as the a phase ispromoted, so that the structural stability at high temperatures isdeteriorated, and the toughness and ductility decrease. Further, theweldability and hot workability are deteriorated. Therefore, the contentof Si is set to 2% or less. In the case where much importance isattached to the toughness and ductility, the content of Si is preferablyset to 1% or less. In the case where the deoxidizing action has beenensured by any other element, it is not necessary to regulate the lowerlimit of the Si content.

In a case where much importance is attached to the deoxidizing action,oxidation resistance, steam oxidation resistance, and the like, thecontent of Si is preferably 0.05% or more, further preferably 0.1% ormore.

Mn: 3% or less

Like Si, Mn (manganese) has a deoxidizing effect. Mn also has the effectof fixing S, which is inevitably contained in the alloy, as sulfides,and therefore Mn does improve the hot workability. However, if the Mncontent exceeds 3%, the precipitation of intermetallic compounds, suchas the σ phase is promoted, so that the structural stability and themechanical properties, such as high temperature strength, aredeteriorated. Therefore, the content of Mn is set to 3% or less.

It is not necessary to regulate the lower limit of the Mn content;however in the case where much importance is attached to the action forimproving hot workability, the content of Mn is preferably set to 0.1%or more. The content of Mn is further preferably set to 0.2 to 2%, stillfurther preferably set to 0.2 to 1.5%.

P: 0.03% or less

P (phosphorus) is inevitably incorporated in the alloy as an impurity,and deteriorates the hot workability. In particular, if the content of Pexceeds 0.03%, the hot workability deteriorates remarkably. Therefore,the content of P is set to 0.03% or less.

In addition to being limited to 0.03% or less, the content of P mustsatisfy the following formula:

P≦3/{200(Ti+8.5×Zr)}  (1).

S: 0.01% or less

Like P, S (sulfur) is inevitably incorporated in the alloy as animpurity, and deteriorates the hot workability. In particular, if thecontent of S exceeds 0.01%, the remarkable deterioration of hotworkability occurs. Therefore, the content of S is set to 0.01% or less.

In the case where it is desired to ensure excellent hot workability, thecontent of S is preferably set to 0.005% or less, further preferably setto 0.003% or less.

Cr: 28 to 38%

Cr (chromium) has the effect of improving the corrosion resistance suchas oxidation resistance, steam oxidation resistance, and hightemperature corrosion resistance. Further, in the present invention, Cris an element that is essential in precipitating as α-Cr phase whichenhances the creep rupture strength. However, if the content of Cr isless than 28%, these effects cannot be obtained. On the other hand, ifthe Cr content increases and especially exceeds 38%, the hot workabilityis deteriorated, and further the structural stability is impaired by theprecipitation of a phase and the like. Therefore, the content of Cr isset to 28 to 38%. An amount more than 30% of Cr content is preferable.

Ni: more than 40% to not more than 60%

Ni (nickel) is an element that is essential in ensuring a stableaustenitic microstructure. In the present invention, since 28 to 38% ofCr is contained, in order to restrain the precipitation of the σ phaseand to stably precipitate α-Cr phase, a content of Ni more than 40% isnecessary. However, if the content of Ni becomes excessive andespecially exceeds 60%, depending on the content of Cr, the α-Cr phasedoes not precipitate sufficiently, and the economic efficiency isdamaged. Therefore, the content of Ni is set to more than 40% to notmore than 60%.

In addition to being limited to more than 40% to not more than 60%, thecontent of Ni must satisfy the following formula:

1.35×Cr≦Ni≦1.85×Cr  (2),

or, in the case where the later-described amount of Co is compositelycontained, the content of Ni must satisfy the following formula:

1.35×Cr≦Ni+Co≦1.85×Cr  (4).

W: more than 3% to not more than 15%

W (tungsten) is a very important element that not only contributes tothe improvement in creep rupture strength as a solid solutionstrengthening element by dissolving into the matrix but alsosignificantly improves the creep rupture strength by precipitating as anFe₂W type Laves phase or an Fe₇W₆ type μ phase. Further, in the presentinvention, since 28 to 38% of Cr is contained, W dissolves into theprecipitated α-Cr phase, restraining the growing and coarsening of α-Crphase during a long period of use at a high temperature, and inhibitinga sudden decrease in creep rupture strength on the long time side.However, if the content of W is 3% or less, the above-described effectscannot be obtained. On the other hand, even if an amount more than 15%of W is contained, the effects saturate and only the cost increases, andmoreover, the structural stability and hot workability are deteriorated.Therefore, the content of W is set to more than 3% to not more than 15%.The content of W is preferably set to more than 3% to not more than 13%.In the case where much importance is further attached to the effect ofimproving the creep rupture strength, the content of W is furtherpreferably set to more than 6% to not more than 13%.

Ti: 0.05 to 1.0%

Ti (titanium) is an important element that promotes the precipitation ofα-Cr phase and thereby enhances the creep rupture strength. Inparticular, by containing Ti compositely with the later-described amountof Zr, the precipitation of α-Cr phase is further promoted, so that thecreep rupture strength can further be enhanced. However, if the contentof Ti is less than 0.05%, sufficient effects cannot be obtained. On theother hand, if the content of Ti exceeds 1.0%, the hot workabilitydeteriorates. Therefore, the content of Ti is set to 0.05 to 1.0%. Thecontent of Ti is preferably set to 0.1 to 0.9%, further preferably setto 0.2 to 0.9%. The still further preferable upper limit of the contentof Ti is 0.5%.

In addition to being limited to 0.05 to 1.0%, the content of Ti mustsatisfy the following formula:

P≦3/{200(Ti+8.5×Zr)}  (1).

Zr: 0.005 to 0.2%

Like Ti, Zr (zirconium) is an important element that promotes theprecipitation of α-Cr phase and thereby enhances the creep rupturestrength. In particular, by containing Zr compositely with theabove-described amount of Ti, the precipitation of α-Cr phase is furtherpromoted, so that the creep rupture strength can further be enhanced.However, if the content of Zr is less than 0.005%, sufficient effectscannot be obtained. On the other hand, if the content of Zr exceeds0.2%, the hot workability deteriorates. Therefore, the content of Zr isset to 0.005 to 0.2%. The content of Zr is preferably set to 0.01 to0.1% and more preferably set to 0.01 to 0.05%.

In addition to being limited to 0.005 to 0.2%, the content of Zr mustsatisfy the following two formulas:

P≦3/{200(Ti+8.5×Zr)}  (1),

Al≧1.5×Zr  (3).

Al: 0.01 to 0.3%

Al (aluminum) is an element having the effect of deoxidizing, and inorder to obtain the said effect, the content of Al should be 0.01% ormore. In the case where much Al is contained, the creep rupture strengthcan be enhanced by the precipitation of Y′ phase. In the presentinvention, however, since the proper amounts of W, Ti and Zr arecontained, and the creep rupture strength can be enhanced dramaticallyby the composite precipitation strengthening due to α-Cr phase, Lavesphase, and the like, the strengthening due to Y′ phase is not necessary.Moreover, in the case where the content of Al exceeds 0.3%, the hotworkability, ductility, and toughness may be deteriorated. Therefore,attaching much importance to hot workability, ductility, and toughness,the content of Al is set to 0.01 to 0.3%.

In addition to being limited to 0.01 to 0.3%, the content of Al mustsatisfy the following formula:

Al≧1.5×Zr  (3).

N: 0.02% or less

In the present invention in which Zr and Ti are contained as essentialelements to promote the precipitation of α-Cr phase, N (nitrogen), whichis an element contained inevitably in the ordinary melting method, mustbe decreased in content as far as possible to avoid the consumption ofZr and Ti caused by the formation of ZrN and TiN. However, an extremedecrease in N content lowers the economic efficiency because of thenecessity of the special melting method and high purity raw material.Therefore, the content of N is set to 0.02% or less. The content of N ispreferably 0.015% or less.

Mo: less than 0.5%

Mo (molybdenum) has conventionally been thought to be an element thatdissolves into the matrix and contributes to the improvement in creeprupture strength as a solid solution strengthening element and that hasthe action equivalent to that of W. However, by the studies of thepresent inventors, it turned out that in the case where Mo iscompositely contained in the alloy containing the above-describedamounts of W and Cr, the σ phase may precipitate on the long time side,and therefore the creep rupture strength, ductility, and toughness maydeteriorate. Consequently, the content of Mo is preferably as low aspossible, and so, the content thereof is set to less than 0.5%. Thecontent of Mo is further preferably limited to less than 0.2%.

One austenitic heat resistant alloy of the present invention comprisesthe above-described elements with the balance being Fe and impurities.Another austenitic heat resistant alloy of the present inventioncontains Co in the amount described below in addition to theabove-described elements.

Co: 20% or less

Like Ni, Co (cobalt) is an element that has the effect of stabilizingthe austenitic microstructure. Co also contributes to the improvement increep rupture strength. And therefore, Co may be contained to obtain theabove-described effects. However, even if the content of Co exceeds 20%,the above-described effects saturate and the cost increases, andmoreover the hot workability is also deteriorated. Therefore, in thecase where Co is contained, the content of Co is set to 20% or less. Theupper limit of the Co content is preferably set to 15%. On the otherhand, in order to ensure the above-described effects of stabilizing theaustenitic microstructure and of improving the creep rupture strengthdue to the Co, the lower limit of the Co content is preferably set to0.05% and more preferably set to 0.5%.

In the case where Co is contained, in addition to being limited to 20%or less, the content of Co must satisfy the following formula:

1.35×Cr≦Ni+Co≦1.85×Cr  (4).

Another austenitic heat resistant alloy of the present invention furthercontains, in addition to the above-described elements of C to Mo or inaddition to the above-described elements of C to Co, one or moreelements of one or more groups selected from the

1

to

3

groups listed below in lieu of a part of Fe:

1

Nb: 1.0% or less, V: 1.5% or less, Hf: 1% or less, and B: 0.05% or less;

2

Mg: 0.05% or less, Ca: 0.05% or less, Y: 0.5% or less, La: 0.5% or less,Ce: 0.5% or less, Nd: 0.5% or less, and Sc: 0.5% or less;

3

Ta: 8% or less, Re: 8% or less, Ir: 5% or less, Pd: 5% or less, Pt: 5%or less, and Ag: 5% or less.

Hereunder, the above-mentioned elements will be explained.

Each of Nb, V, Hf and B being elements of the

1

group, has the effects of enhancing the high temperature strength andcreep rupture strength. Therefore, in the case where it is desired toobtain the enhanced high temperature strength and creep rupturestrength, these elements are added positively, and one or more elementsamong them may be contained in the range described below.

Nb: 1.0% or less

Nb (niobium) has the effects of enhancing the high temperature strengthand creep rupture strength by forming carbo-nitrides and also itimproves the ductility by making the grains fine. Therefore, in order toobtain these effects, Nb may be contained. However, if the content of Nbexceeds 1.0%, the hot workability and toughness are deteriorated.Therefore, in the case where Nb is contained, the content of Nb is setto 1.0% or less. The upper limit of the Nb content is preferably set to0.9%. On the other hand, in order to ensure the above-described effectsof enhancing the high temperature strength, creep rupture strength, andductility due to Nb, the lower limit of the Nb content is preferably setto 0.05% and further preferably set to 0.1%.

V: 1.5% or less

V (vanadium) has the effects of enhancing the high temperature strengthand creep rupture strength by forming carbo-nitrides. Therefore, inorder to obtain these effects, V may be contained. However, if thecontent of V exceeds 1.5%, the high temperature corrosion resistance isdeteriorated, and further the ductility and toughness are decreased dueto the precipitation of brittle phase. Therefore, in the case where V iscontained, the content of V is set to 1.5% or less. The upper limit ofthe V content is preferably set to 1%. On the other hand, in order toensure the above-described effects of enhancing the high temperaturestrength and creep rupture strength due to V, the lower limit of the Vcontent is preferably set to 0.02% and more preferably set to 0.04%.

Hf: 1% or less

Hf (hafnium) contributes to precipitation strengthening as acarbo-nitride and has the effects of enhancing the high temperaturestrength and creep rupture strength. Therefore, in order to obtain theseeffects, Hf may be contained. However, if the content of Hf exceeds 1%,the workability and weldability are impaired. Therefore, in the casewhere Hf is contained, the content of Hf is set to 1% or less. The upperlimit of the Hf content is preferably set to 0.8% and more preferablyset to 0.5%. On the other hand, in order to ensure the above-describedeffects of enhancing the high temperature strength and creep rupturestrength due to Hf, the lower limit of the Hf content is preferably setto 0.01% and further preferably set to 0.02%.

B: 0.05% or less

B (boron) exists at grain boundaries as a single form or it exists incarbo-nitrides. B has the effects of enhancing the high temperaturestrength and creep rupture strength by restraining a grain boundary slipcaused by grain boundary strengthening during the use at a hightemperature and also by promoting the fine dispersing precipitation ofcarbo-nitrides. However, if the content of B exceeds 0.05%, theweldability is deteriorated. Therefore, in the case where B iscontained, the content of B is set to 0.05% or less. The upper limit ofthe B content is preferably set to 0.01% and more preferably set to0.005%. On the other hand, in order to ensure the above-describedeffects of enhancing the high temperature strength and creep rupturestrength due to B, the lower limit of the B content is preferably set to0.0005% and further preferably set to 0.001%.

The upper limit of the sum of the contents of the above-describedelements from Nb to B may be 3.55%. The upper limit of the sum ofcontents thereof is further preferably 2.5%.

Each of Mg, Ca, Y, La, Ce, Nd and Sc being elements of the

2

group, has the effect of improving the hot workability by fixing S assulfides. Therefore, in the case where it is desired to obtain furtherexcellent hot workability, these elements are added positively, and oneor more elements among them may be contained in the range describedbelow.

Mg: 0.05% or less

Mg (magnesium) has the effect of improving the hot workability by fixingS, which is contained inevitably in the alloy, as sulfides. Therefore,in order to obtain this effect, Mg may be contained. However, if thecontent of Mg exceeds 0.05%, the cleanliness of the alloy isdeteriorated, and the hot workability and ductility are contrarilyimpaired. Therefore, in the case where Mg is contained, the content ofMg is set to 0.05% or less. The upper limit of the Mg content ispreferably set to 0.02% and more preferably set to 0.01%. On the otherhand, in order to ensure the above-described effect of improving the hotworkability due to Mg, the lower limit of the Mg content is preferablyset to 0.0005% and further preferably set to 0.001%.

Ca: 0.05% or less

Ca (calcium) has the effect of improving the hot workability by fixingS, which inhibits the hot workability, as sulfides. Therefore, in orderto obtain this effect, Ca may be contained, however, if the content ofCa exceeds 0.05%, the cleanliness of the alloy is deteriorated, and thehot workability and ductility are contrarily impaired. Therefore, in thecase where Ca is contained, the content of Ca is set to 0.05% or less.The upper limit of the Ca content is preferably set to 0.02% and morepreferably set to 0.01%. On the other hand, in order to ensure theabove-described effect of improving the hot workability due to Ca, thelower limit of the Ca content is preferably set to 0.0005% and furtherpreferably set to 0.001%.

Y: 0.5% or less

Y (yttrium) has the effect of improving the hot workability by fixing Sas sulfides. Y also has the effect of improving the adhesiveness of aCr₂O₃ protective film on the alloy surface, especially improving theoxidation resistance at the time of repeated oxidation, and further Yhas the effects of enhancing the creep rupture strength and creeprupture ductility by contributing to grain boundary strengthening.However, if the content of Y exceeds 0.5%, the amounts of inclusions,such as oxides increase, so that the workability and weldability areimpaired. Therefore, in the case where Y is contained, the content of Yis set to 0.5% or less. The upper limit of the Y content is preferablyset to 0.3% and further preferably set to 0.15%. On the other hand, inorder to ensure the above-described effects due to Y, the lower limit ofthe Y content is preferably set to 0.0005%. The lower limit of the Ycontent is more preferably 0.001% and still more preferably 0.002%.

La: 0.5% or less

La (lanthanum) has the effect of improving the hot workability by fixingS as sulfides. La also has the effect of improving the adhesiveness of aCr₂O₃ protective film on the alloy surface, especially improving theoxidation resistance at the time of repeated oxidation, and further Lahas the effects of enhancing the creep rupture strength and creeprupture ductility by contributing to grain boundary strengthening.However, if the content of La exceeds 0.5%, the amounts of inclusions,such as oxides increase, so that the workability and weldability areimpaired. Therefore, in the case where La is contained, the content ofLa is set to 0.5% or less. The upper limit of the La content ispreferably set to 0.3% and further preferably set to 0.15%. On the otherhand, in order to ensure the above-described effects due to La, thelower limit of the La content is preferably set to 0.0005%. The lowerlimit of the La content is more preferably 0.001% and still morepreferably 0.002%.

Ce: 0.5% or less

Ce (cerium) also has the effect of improving the hot workability byfixing S as sulfides. In addition, Ce has the effect of improving theadhesiveness of a Cr₂O₃ protective film on the alloy surface, especiallyimproving the oxidation resistance at the time of repeated oxidation,and further Ce has the effects of enhancing the creep rupture strengthand creep rupture ductility by contributing to grain boundarystrengthening. However, if the content of Ce exceeds 0.5%, the amountsof inclusions, such as oxides increase, so that the workability andweldability are impaired. Therefore, in the case where Ce is contained,the content of Ce is set to 0.5% or less. The upper limit of the Cecontent is preferably set to 0.3% and further preferably set to 0.15%.On the other hand, in order to ensure the above-described effects due toCe, the lower limit of the Ce content is preferably set to 0.0005%. Thelower limit of the Ce content is more preferably 0.001% and still morepreferably 0.002%.

Nd: 0.5% or less

Nd (neodymium) has the effect of improving the hot workability by fixingS as sulfides. Nd also has the effect of improving the adhesiveness of aCr₂O₃ protective film on the alloy surface, especially improving theoxidation resistance at the time of repeated oxidation, and further Ndhas the effects of enhancing the creep rupture strength and creeprupture ductility by contributing to grain boundary strengthening.However, if the content of Nd exceeds 0.5%, the amounts of inclusions,such as oxides increase, so that the workability and weldability areimpaired. Therefore, in the case where Nd is contained, the content ofNd is set to 0.5% or less. The upper limit of the Nd content ispreferably set to 0.3% and further preferably set to 0.15%. On the otherhand, in order to ensure the above-described effects due to Nd, thelower limit of the Nd content is preferably set to 0.0005%. The lowerlimit of the Nd content is more preferably 0.001% and still morepreferably 0.002%.

Sc: 0.5% or less

Sc (scandium) also has the effect of improving the hot workability byfixing S as sulfides. In addition, Sc has the effect of improving theadhesiveness of a Cr₂O₃ protective film on the alloy surface, especiallyimproving the oxidation resistance at the time of repeated oxidation,and further Sc has the effects of enhancing the creep rupture strengthand creep rupture ductility by contributing to grain boundarystrengthening. However, if the content of Sc exceeds 0.5%, the amountsof inclusions, such as oxides increase, so that the workability andweldability are impaired. Therefore, in the case where Sc is contained,the content of Sc is set to 0.5% or less. The upper limit of the Sccontent is preferably set to 0.3% and further preferably set to 0.15%.On the other hand, in order to ensure the above-described effects due toSc, the lower limit of the Sc content is preferably set to 0.0005%. Thelower limit of the Sc content is more preferably 0.001% and still morepreferably 0.002%.

The upper limit of the sum of contents of the above-described elementsfrom Mg to Sc may be 2.6%. The upper limit of the sum of contentsthereof is further preferably 1.5%.

Each of Ta, Re, Ir, Pr, Pt and Ag being elements of the (3) group, hasthe effect of solid solution strengthening by dissolving into theaustenite, which is the matrix. Therefore, in a case where it is desiredto obtain far higher strength by the solid solution strengtheningaction, these elements are added positively, and one or more elementsamong them may be contained in the range described below.

Ta: 8% or less

Ta (tantalum) has the effects of enhancing the high temperature strengthand creep rupture strength by dissolving into the austenite, which isthe matrix, and by forming carbo-nitrides. Therefore, in order to obtaintheses effects, Ta may be contained. However, if the content of Taexceeds 8%, the workability and mechanical properties are impaired.Therefore, in the case where Ta is contained, the content of Ta is setto 8% or less. The upper limit of the Ta content is preferably set to 7%and more preferably set to 6%. On the other hand, in order to ensure theabove-described effects due to Ta, the lower limit of the Ta content ispreferably set to 0.01%. The lower limit of the Ta content is morepreferably 0.1% and still more preferably 0.5%.

Re: 8% or less

Re (rhenium) has the effects of enhancing the high temperature strengthand creep rupture strength by dissolving into the austenite, which isthe matrix. Therefore, in order to obtain theses effects, Re may becontained. However, if the Re content exceeds 8%, the workability andmechanical properties are impaired. Therefore, in the case where Re iscontained, the content of Re is set to 8% or less. The upper limit ofthe Re content is preferably set to 7% and more preferably set to 6%. Onthe other hand, in order to ensure the above-described effects due toRe, the lower limit of the Re content is preferably set to 0.01%. Thelower limit of the Re content is more preferably 0.1% and still morepreferably 0.5%.

Ir: 5% or less

Ir (iridium) has the effects of enhancing the high temperature strengthand creep rupture strength by dissolving into the austenite, which isthe matrix, and by forming fine intermetallic compounds according to thecontent. Therefore, in order to obtain theses effects, Ir may becontained. However, if the Ir content exceeds 5%, the workability andmechanical properties are impaired. Therefore, in the case where Ir iscontained, the content of Ir is set to 5% or less. The upper limit ofthe Ir content is preferably set to 4% and more preferably set to 3%. Onthe other hand, in order to ensure the above-described effects due toIr, the lower limit of the Ir content is preferably set to 0.01%. Thelower limit of the Ir content is more preferably 0.05% and still morepreferably 0.1%.

Pd: 5% or less

Pd (palladium) has the effects of enhancing the high temperaturestrength and creep rupture strength by dissolving into the austenite,which is the matrix, and by forming fine intermetallic compoundsaccording to the content. Therefore, in order to obtain theses effects,Pd may be contained. However, if the Pd content exceeds 5%, theworkability and mechanical properties are impaired. Therefore, in thecase where Pd is contained, the content of Pd is set to 5% or less. Theupper limit of the Pd content is preferably set to 4% and morepreferably set to 3%. On the other hand, in order to ensure theabove-described effects due to Pd, the lower limit of the Pd content ispreferably set to 0.01%. The lower limit of the Pd content is morepreferably 0.05% and still more preferably 0.1%.

Pt: 5% or less

Pt (platinum) also has the effects of enhancing the high temperaturestrength and creep rupture strength by dissolving into the austenite,which is the matrix, and by forming fine intermetallic compoundsaccording to the content. Therefore, in order to obtain theses effects,Pt may be contained. However, if the Pt content exceeds 5%, theworkability and mechanical properties are impaired. Therefore, in thecase where Pt is contained, the content of Pt is set to 5% or less. Theupper limit of the Pt content is preferably set to 4% and morepreferably set to 3%. On the other hand, in order to ensure theabove-described effects due to Pt, the lower limit of the Pt content ispreferably set to 0.01%. The lower limit of the Pt content is morepreferably 0.05% and still more preferably 0.1%.

Ag: 5% or less

Ag (silver) has the effects of enhancing the high temperature strengthand creep rupture strength by dissolving into the austenite, which isthe matrix, and by forming fine intermetallic compounds according to thecontent. Therefore, in order to obtain theses effects, Ag may becontained. However, if the Ag content exceeds 5%, the workability andmechanical properties are impaired. Therefore, in the case where Ag iscontained, the content of Ag is set to 5% or less. The upper limit ofthe Ag content is preferably set to 4% and more preferably set to 3%. Onthe other hand, in order to ensure the above-described effects due toAg, the lower limit of the Ag content is preferably set to 0.01%. Thelower limit of the Ag content is more preferably 0.05% and still morepreferably 0.1%.

The sum of contents of the above-described elements from Ta to Ag ispreferably 10% or less. The upper limit of the sum of contents thereofis further preferably 8%.

P≦3/{200(Ti+8.5×Zr)}

Regarding the austenitic heat resistant alloy of the present invention,the contents of Ti, Zr and P each must be in an already-described range,and also must satisfy the following formula:

P≦3/{200(Ti+8.5×Zr)}  (1).

The reason for this is as follows. Since Ti and Zr lower the meltingpoint of the heat resistant alloy, and P deteriorates the hotworkability, in the case where the contents of Ti, Zr and P are in thealready-described ranges respectively but do not satisfy the aboveformula (1), the hot workability, especially the hot workability on thehigh temperature side of 1150° C. or higher deteriorates, and furtherthe high temperature crack resistance at the time of welding maydeteriorate. However, if the contents of Ti, Zr and P satisfy theabove-described formula (1), the hot workability on the high temperatureside of 1150° C. or higher can be improved stably and reliably, whilethe high creep rupture strength is maintained, and further, the hightemperature crack resistance at the time of welding can also beimproved.

1.35×Cr≦Ni≦1.85×Cr, or 1.35×Cr≦Ni+Co≦1.85×Cr

In a case where the content of Ni is in the already-described range andsatisfies the following formula,

1.35×Cr≦Ni≦1.85×Cr  (2)

in relation to the Cr content, or, in a case where Co is compositelycontained and the contents of both Ni and Co are in thealready-described range and satisfy the following formula,

1.35×Cr≦Ni+Co≦1.85×Cr  (4)

in relation to the Cr content, the precipitation of a phase during along period of use at a high temperature can be restrained stably andreliably, and moreover, the optimum amount of the α-Cr phase can beprecipitated. Therefore, the austenitic heat resistant alloy of thepresent invention is regulated to satisfy the formula (2) or formula(4).

Al≧1.5×Zr

Regarding the austenitic heat resistant alloy of the present invention,the content of Al and Zr must be in the already-described range, andalso must satisfy the following formula:

Al≧1.5×Zr  (3).

The reason for this is that in a case where the contents of Al and Zr donot satisfy formula (3), though being in the already-described range, insome cases, the action of Zr for promoting the precipitation of the α-Crphase to enhance the creep rupture strength cannot be ensuredsufficiently. However, if the contents of Al and Zr satisfy formula (3),the action of Zr for promoting the precipitation of the α-Cr phase toenhance the creep rupture strength can be performed stably and reliably.

As described above, the austenitic heat resistant alloy of the presentinvention is excellent in creep resistance properties and structuralstability. Therefore, if this austenitic heat resistant alloy is used asa starting material, a heat resistant pressure member excellent in creepresistance and structural stability in a high temperature range inaccordance with the present invention, can be obtained easily. Theaustenitic heat resistant alloy of the present invention used as thestarting material for the heat resistant pressure member of the presentinvention may be melted and cast in the same way as that of the ordinaryaustenitic alloy.

(B) Method for Manufacturing a Heat Resistant Pressure Member

Next, a preferred method for manufacturing the heat resistant pressuremember, which is made from the austenitic heat resistant alloy of thepresent invention is explained. This manufacturing method has thefeature of including the before-described steps (i), (ii) and (iii)performed in sequence.

Step (i): heating to 1050 to 1250° C. at least once before final hot orcold working

In the method in accordance with the present invention, it is necessaryto dissolve the precipitates in the alloy which precipitated during theworking sufficiently, by heating at least once before the final hot orcold working. However, in the case where the heating temperature islower than 1050° C., undissolved carbo-nitrides and/or oxides, whichcontain Ti and B, come to exist stably in the heated alloy. As a result,the existence thereof results in the accumulation of nonuniform strainin the next step (ii), and makes the recrystallization nonuniform in thefinal heat treatment of the step (iii). Moreover, the said undissolvedcarbo-nitrides and oxides themselves hinder a uniform recrystallization.On the other hand, if the heating is performed at a temperature morethan 1250° C., high temperature intergranular fracture and lowering ofductility may be caused. Therefore, in the preferred method of thepresent invention, heating to 1050 to 1250° C. is performed at leastonce before the final hot or cold working. The preferable lower limit ofthe heating temperature is 1150° C., and the preferable upper limitthereof is 1230° C.

Step (ii): carrying out a final hot or cold plastic working such thatthe reduction of area is 10% or more

The plastic working in step (ii) is carried out to give strains forpromoting recrystallization in the next final heat treatment. In thecase where the reduction of area is less than 10% in this working, astrain necessary for recrystallization cannot be obtained. Therefore,the plastic working is carried out so that the reduction of area is 10%or more. The preferable lower limit of the reduction of area is 20%.Since a larger reduction of area is better, the upper limit thereof isnot defined; however, the maximum value thereof in the ordinary workingis about 90%. This working step is a step that determines the size ofproduct.

In the case where the final working after heating is a hot working, thefinish temperature of the hot working is preferably set to 1000° C. orhigher in order to avoid nonuniform deformation in the temperature rangein which carbides precipitate. Moreover, the cooling condition afterworking is not subject to any special restriction; however, after thefinish of the hot working, in order to restrain the precipitation ofcoarse carbo-nitrides, it is desirable to perform cooling at the highestpossible cooling rate of 0.25° C./s or higher in the temperature rangedown to 500° C.

In the case where the working after heating is a cold working, the coldworking may be performed once as the final working or may be performed anumber of times. In the case where the cold working is performed anumber of times, a cold working is performed after intermediate heattreatment, and the heat treatment temperature in the step (i) and thereduction of area of cold working in the step (ii) have only to besatisfied in the final cold working and in the previous intermediateheat treatment.

Step carrying out a final heat treatment in which cooling is performedafter heating and holding at a temperature in the range of 1100 to 1250°C.

If the heating temperature of this heat treatment is lower than 1100°C., a sufficient recrystallization does not occur. Moreover, grainsbecome depressed working microstructures, so that the creep strengthdecreases. On the other hand, if heating is performed to a temperaturemore than 1250° C., high temperature intergranular fracture and loweringof ductility may be caused, and therefore, the temperature of the finalproduct heat treatment is 1100 to 1250° C. The preferable heat treatmenttemperature is a temperature 10° C. or more higher than the heatingtemperature in the step (i).

The heat resistant pressure member of the present invention need not bemade of a fine grain microstructure from the viewpoint of corrosionresistance. When it is desired to make the heat resistant pressuremember a fine grain microstructure, the final heat treatment has only tobe performed at a temperature of 10° C. or lower than the hot workingfinish temperature or at a temperature of 10° C. or lower than theabove-described intermediate heat treatment temperature. After thisfinal heat treatment, in order to restrain the precipitation of coarsecarbo-nitrides, cooling is preferably performed at the highest possiblecooling rate of 1° C./s or higher.

The following examples illustrate the present invention morespecifically. These examples are, however, by no means limited to thescope of the present invention.

EXAMPLES

Austenitic alloys 1 to 17 and A to K, having the chemical compositionsshown in Table 1, were melted by using a high-frequency vacuum meltingfurnace and cast to form 17 kg ingots each having an outside diameter of100 mm.

The alloys 1 to 17 shown in Table 1 are alloys whose chemicalcompositions fall within the range regulated by the present invention.On the other hand, the alloys A to K are alloys of comparative exampleswhose chemical composition are out of the range regulated by the presentinvention. Both of the alloys G and H are alloys in which the individualcontents of Ni and Co are within the range regulated by the presentinvention, the value of “Ni+Co” does not satisfy the said formula (4).The alloy I is an alloy whose Al content of 0.03% is within the range of“0.01 to 0.3%” which is regulated by the present invention; but the saidcontent of Al does not satisfy the formula (3). The alloy K is an alloywhose P content of 0.009% is within the range of “0.03 or less” which isregulated by the present invention; however the said content of P doesnot satisfy the formula (1).

TABLE 1 Table 1 Chemical composition (% by mass) Balance: Fe andimpurities Alloy C Si Mn P S Cr Ni Co Ni + Co Mo W Ti Al  1 0.057 0.431.05 0.011 0.003 29.2 47.8 — 47.8 — 4.3 0.43 0.12  2 0.059 0.41 1.070.008 0.002 31.3 50.2 — 50.2 — 7.9 0.72 0.16  3 0.056 0.44 1.11 0.0050.002 31.0 53.4 — 53.4 — 11.6  0.53 0.21  4 0.062 0.89 0.95 0.012 0.00335.2 56.5 — 56.5 — 6.8 0.71 0.08  5 0.059 0.37 1.20 0.006 0.002 30.443.4 7.2 50.6 — 8.1 0.82 0.25  6 0.060 0.43 1.08 0.011 0.001 35.8 42.414.5  66.9 — 10.5  0.65 0.14  7 0.055 0.41 0.41 0.008 0.003 30.5 50.3 —50.3 — 7.6 0.74 0.13  8 0.061 0.38 1.85 0.012 0.002 30.2 51.0 — 51.0 —8.5 0.55 0.14  9 0.055 0.50 1.02 0.025 0.0004 34.8 54.9 — 54.9 — 6.60.19 0.11 10 0.089 0.39 1.08 0.010 0.002 29.7 50.3 — 50.3 — 7.8 0.590.12 11 0.058 0.42 1.05 0.006 0.002 30.4 50.1 — 50.1 — 8.0 0.81 0.17 120.134 0.41 1.12 0.005 0.003 30.8 50.5 — 50.5 — 7.5 0.74 0.20 18 0.0740.44 1.17 0.011 0.001 35.7 58.1 — 58.1 — 6.8 0.77 0.10 14 0.056 1.230.34 0.010 0.002 30.2 51.2 — 51.2 — 5.4 0.70 012 15 0.059 0.50 1.520.010 0.002 30.5 50.7 — 50.7 — 7.1 0.85 0.15 16 0.061 0.45 1.10 0.0080.001 30.9 50.6 — 50.6 — 8.1 0.75 0.14 17 0.058 0.47 1.08 0.007 0.00231.2 50.4 — 50.4 — 7.7 0.70 0.16 A 0.061 0.40 1.01 0.007 0.002 31.0 49.9— 49.9 — 8.0 0.75 0.16 B 0.057 0.43 1.06 0.007 0.002 31.2 50.1 — 50.1 —8.1 * — 0.14 C 0.060 0.44 1.07 0.010 0.003 30.1 48.1 — 48.1 — * 2.7 0.45 0.14 D 0.060 0.41 1.01 0.010 0.002 31.5 49.9 — 49.9 — 8.0 0.74 0.15E 0.062 0.41 1.10 0.008 0.003 31.1 50.5 — 50.5 * 2.5 * — 0.75 0.14 F0.061 0.47 0.99 0.010 0.002 31.0 50.4 — 50.4 * 2.2 3.4 0.72 0.16 G 0.0570.37 1.18 0.008 0.003 32.0 40.2 2.4 * 42.6  — 7.5 0.78 0.13 H 0.059 0.391.15 0.006 0.002 29.2 52.1 7.3 * 59.4  — 8.1 0.84 0.25 I 0.060 0.43 1.070.009 0.003 31.1 50.5 — 50.5 — 8.2 0.74 * 0.03  J 0.062 0.43 1.10 0.0110.002 31.0 50.7 — 50.7 — 7.8 0.71 * 0.64  K 0.061 0.38 1.17 * 0.009 0.002 30.2 44.5 7.6 52.1 — 7.8 0.89 0.25 Chemical composition (% bymass) Balance: Fe and impurities Alloy N Zr Others value of f1 value off2 value of f3  1 0.009 0.04 — 0.019 1.637 0.060  2 0.013 0.05 — 0.0181.604 0.075  3 0.011 0.13 — 0.009 1.723 0.195  4 0.012 0.02 B: 0.00630.017 1.605 0.030  5 0.008 0.13 — 0.008 1.664 0.195  6 0.014 0.03 B:0.0041 0.017 1.589 0.045  7 0.012 0.02 V: 0.78, Nb: 0.32, B: 0.00260.016 1.649 0.030  8 0.011 0.04 B: 0.0033, Mg: 0.0023, Ca: 0.0028 0.0171.689 0.060  9 0.010 0.02 B: 0.0043, Y: 0.02, La: 0.03 0.042 1.578 0.03010 0.012 0.03 Nd: 0.03 0.018 1.694 0.045 11 0.005 0.03 Ce: 0.03, Sc:0.05 0.014 1.648 0.045 12 0.007 0.05 Hf: 0.28, Re: 1.2 0.013 1.640 0.07518 0.005 0.02 Ta: 1.3 0.016 1.627 0.030 14 0.012 0.03 Ir: 1.2, Ag: 1.50.016 1.695 0.045 15 0.008 0.02 Pd: 1.1, Pt: 1.0 0.015 1.662 0.030 160.013 0.04 Ca: 0.0035, Ta: 3.8 0.014 1.638 0.060 17 0.012 0.03 B:0.0031, Mg: 0.0041, Re: 2.4 0.016 1.615 0.045 A 0.013 * — — 0.020 1.610— B 0.013 0.06 — 0.029 1.606 0.090 C 0.010 0.04 — 0.019 1.598 0.060 D *0.024  0.05 — 0.013 1.584 0.075 E 0.012 0.04 — 0.014 1.624 0.060 F 0.0130.05 — 0.013 1.626 0.075 G 0.007 0.04 — 0.013 1.331 0.060 H 0.008 0.12 —0.008 2.034 0.180 I 0.014 0.05 — 0.013 1.624 0.075 J 0.013 0.04 — 0.0141.635 0.060 K 0.008 0.15 — 0.007 1.725 0.225 f1 = 3/{200(Ti + 8.5 ×Zr)}, f2 = (Ni + Co)/Cr, f3 = 1.5 × Zr The mark * indicates fallingoutside the conditions regulated by the present invention.

Thus the obtained ingot was heated to 1180° C., and then was hot forgedso that the finish temperature was 1050° C. to form a plate materialhaving a thickness of 15 mm. After the hot forging, the plate materialwas air cooled.

From a middle portion in the thickness direction of the 15 mm thickplate material obtained by the above-mentioned hot forging, a round bartensile test specimen, having a diameter of 10 mm and a length of 130mm, was produced by machining the plate material in parallel to thelongitudinal direction, and the tensile test specimen was used toevaluate the high temperature ductility.

That is to say, the said round bar tensile test specimen was heated to1200° C. and was held for 3 minutes, and then a high speed tensile testwas conducted at a strain rate of 10/s in order to determine thereduction of area from the fracture surface after testing. It was foundthat if the reduction of area is 60% or more, no major problem occurred,even if hot working, such as hot extrusion is performed at thattemperature. Therefore, the reduction of area of “60% or more” was madethe criterion of excellent hot workability.

Moreover, using the 15 mm thick plate material obtained by the said hotforging, a softening heat treatment was performed at 1100° C., and thenthe plate material was cold rolled so that the thickness thereof becomes10 mm, and further, the cold rolled plate material was water cooledafter being held at 1200° C. for 30 minutes.

Using a part of the above-described 10 mm thick plate material watercooled after being held at 1200° C. for 30 minutes, and from a middleportion in the thickness direction of the part, a round bar tensile testspecimen, having a diameter of 6 mm and a gage length of 30 mm, wasproduced by machining the part in parallel to the longitudinaldirection; the tensile test specimen was used to conduct a creep rupturetest.

That is to say, by using the above-described test specimen, the creeprupture test was conducted in the air of 700° C., 750° C. and 800° C.,and by generalizing the obtained rupture strength using theLarson-Miller parameter method, the rupture strength at 700° C. in10,000 hours was determined.

Furthermore, the remainder of the 10 mm thick plate material watercooled after being held at 1200° C. for 30 minutes was subjected to anaging treatment in which the test specimen was held at 750° C. for 5000hours, and then was water cooled.

From a middle portion in the thickness direction of the 10 mm thickplate material water cooled after an aging treatment, a V-notch testspecimen having a width of 5 mm, a height of 10 mm, and a length of 55mm, specified in JIS Z 2242 (2005) was produced in parallel to thelongitudinal direction, and a Charpy impact test at 0° C. was conductedon the test specimen in order to measure the impact value and evaluatethe toughness.

The results of the above-described tests are summarized in Table 2.

TABLE 2 Creep rupture strength Reduction at 700° C. × Charpy of areaTest 10000 h impact value at 1200° C. No. Alloy (MPa) (J/cm²) (%) Note 11 158.4 63.4 86.5 Inventive 2 2 165.2 55.6 80.4 example 3 3 168.5 48.385.2 4 4 170.1 57.4 80.8 5 5 169.3 51.2 71.5 6 6 172.2 41.3 72.6 7 7169.5 53.5 81.0 8 8 164.1 56.2 88.7 9 9 155.3 59.0 92.5 10 10 163.5 57.890.1 11 11 166.4 55.9 85.6 12 12 165.0 56.0 81.2 13 13 171.2 57.6 81.114 14 165.4 58.5 81.9 15 15 167.2 53.7 71.8 16 16 168.3 54.8 86.2 17 17167.9 55.0 87.1 18 *A 142.5 55.8 80.8 Comparative 19 *B 135.1 55.1 85.3example 20 *C 148.9 63.9 86.2 21 *D 151.5 51.6 78.8 22 *E 141.1 11.580.6 23 *F 143.5 13.4 81.0 24 *G 148.5 15.2 79.8 25 *H 139.6 52.8 71.126 *I 151.9 51.9 81.2 27 *J 164.9 24.8 52.3 28 *K 169.0 50.7 50.2 Themark * indicates falling outside the conditions regulated by the presentinvention.

From Table 2, regarding the test Nos. 1 to 17 using the alloys 1 to 17,which are the inventive examples, it is apparent that all of the creeprupture strength, toughness after aging, and hot workability areexcellent.

In contrast, regarding the test Nos. 18 to 28 using the alloys A to K,which are the comparative examples deviating from the conditionsregulated by the present invention, at least one of the creep rupturestrength, toughness after aging, and hot workability is poorer than thatof the above-mentioned test Nos. 1 to 17, being the inventive examples

That is to say, in the case of test No. 18, the chemical composition ofthe alloy A is almost equivalent to that of the alloy 2, used in thetest No. 2. However, the said alloy A does not contain Zr, and thereforethe creep rupture strength is low.

In the case of test No. 19, the chemical composition of the alloy B isalmost equivalent to that of the alloy 2, used in the test No. 2.However, the said alloy B does not contain Ti, and therefore the creeprupture strength is low.

In the case of test No. 20, the chemical composition of the alloy C isalmost equivalent to that of the alloy 1, used in the test No. 1.However, the W content of the said alloy C is “2.7%”, which is lowerthan the value regulated by the present invention, and therefore thecreep rupture strength is low.

In the case of test No. 21, the chemical composition of the alloy D isalmost equivalent to that of the alloy 2, used in the test No. 2.However, the N content of the said alloy D is “0.024%”, which is higherthan the value regulated by the present invention, and therefore thecreep rupture strength is low.

In the case of test No. 22, the chemical composition of the alloy E isalmost equivalent to that of the alloy 2, used in the test No. 2.However the said alloy E does not contain W, and moreover the Mo contentthereof is “2.5%”, which is higher than the value regulated by thepresent invention. Therefore, the creep rupture strength is low, andfurther the Charpy impact value after aging is remarkably low, so thatthe toughness is poor.

In the case of test No. 23, if the operational advantage of W is about ahalf of that of Mo, that is to say, if the W content corresponds toabout a half of the Mo content, as being said conventionally, the alloyF is an alloy which is equivalent to the alloy 2, used in the test No.2. However, the Mo content of the said alloy F is “2.2%”, which exceedsthe value regulated by the present invention. Therefore, the creeprupture strength is low, and further the Charpy impact value after agingis remarkably low, so that the toughness is poor.

In the case of test No. 24, the chemical composition of the alloy G isalmost equivalent to that of the alloy 5, used in the test No. 5.However the sum of the Ni content and the Co content, that is to say,the value of “Ni+Co” of the said alloy G is lower than “1.35×Cr” anddoes not satisfy the formula (4). Therefore, the creep rupture strengthis low, and moreover the Charpy impact value after aging is remarkablylow, so that the toughness is poor.

In the case of test No. 25, the chemical composition of the alloy H isalmost equivalent to that of the alloy 5, used in the test No. 5.However, the sum of the Ni content and the Co content, that is to say,the value of “Ni+Co” of the said alloy H is higher than “1.85×Cr” anddoes not satisfy the formula (4). Therefore, the creep rupture strengthis low.

In the case of test No. 26, the chemical composition of the alloy I isalmost equivalent to that of the alloy 2, used in the test No. 2.However, the Al content of the said alloy I is lower than “1.5×Zr” anddoes not satisfy the formula (3). Therefore, the creep rupture strengthis low.

In the case of test No. 27, the chemical composition of the alloy J isalmost equivalent to that of the alloy 2, used in test No. 2. However,the Al content of the said alloy J is “0.64%”, which is higher than thevalue regulated by the present invention. Therefore, the Charpy impactvalue after aging is remarkably low, so that the toughness is poor.Moreover the reduction of area at 1200° C. does not reach 60%, so thatthe hot workability is low.

In the case of test No. 28, the chemical composition of the alloy K isalmost equivalent to that of the alloy 5, used in the test No. 5.However, the P content of the said alloy K exceeds “3/{200(Ti+8.5×Zr)}”and does not satisfy the formula (1). Therefore, the reduction of areaat 1200° C. is 50.2%, so that the hot workability is remarkably low.

INDUSTRIAL APPLICABILITY

The austenitic heat resistant alloy according to the present invention,has high temperature strength, especially creep rupture strength, higherthan that of the conventional heat resistant alloys, and also has hightoughness because the structural stability is excellent even after along period of use at a high temperature. Further it is excellent in hotworkability, especially high temperature ductility at 1150° C. orhigher. Therefore, this austenitic heat resistant alloy can be suitablyused as a pipe material, a plate material for a heat resistant pressuremember, a bar material, forgings, and the like for a boiler for powergeneration, a plant for chemical industry and so on.

1. An austenitic heat resistant alloy, which comprises by mass percent,C: more than 0.02% to not more than 0.15%, Si: 2% or less, Mn: 3% orless, P: 0.03% or less, S: 0.01% or less, Cr: 28 to 38%, Ni: more than40% to not more than 60%, W: more than 3% to not more than 15%, Ti: 0.05to 1.0%, Zr: 0.005 to 0.2%, Al: 0.01 to 0.3%, N: 0.02% or less, and Mo:less than 0.5%, with the balance being Fe and impurities, in which thefollowing formulas (1) to (3) are satisfied:P≦3/{200(Ti+8.5×Zr)}  (1),1.35×Cr≦Ni≦1.85×Cr  (2),Al≧1.5×Zr (3); wherein each element symbol in the equations (1) to (3)represents the content by mass % of the element concerned.
 2. Anaustenitic heat resistant alloy, which comprises by mass percent, C:more than 0.02% to not more than 0.15%, Si: 2% or less, Mn: 3% or less,P: 0.03% or less, S: 0.01% or less, Cr: 28 to 38%, Ni: more than 40% tonot more than 60%, Co: 20% or less, W: more than 3% to not more than15%, Ti: 0.05 to 1.0%, Zr: 0.005 to 0.2%, Al: 0.01 to 0.3%, N: 0.02% orless, and Mo: less than 0.5%, with the balance being Fe and impurities,in which the following formulas (1), (3) and (4) are satisfied:P≦3/{200(Ti+8.5×Zr)}  (1),Al≧1.5×Zr  (3),1.35×Cr≦Ni+Co≦1.85×Cr  (4); wherein each element symbol in the equations(1), (3) and (4) represents the content by mass % of the elementconcerned.
 3. The austenitic heat resistant alloy according to claim 1,which further contains, by mass percent, one or more elements of one ormore groups selected from the

1

to

3

groups listed below in lieu of a part of Fe:

1

Nb: 1.0% or less, V: 1.5% or less, Hf: 1% or less and B: 0.05% or less;

2

Mg: 0.05% or less, Ca: 0.05% or less, Y: 0.5% or less, La: 0.5% or less,Ce: 0.5% or less, Nd: 0.5% or less and Sc: 0.5% or less;

3

Ta: 8% or less, Re: 8% or less, Ir: 5% or less, Pd: 5% or less, Pt: 5%or less and Ag: 5% or less.
 4. The austenitic heat resistant alloyaccording to claim 2, which further contains, by mass percent, one ormore elements of one or more groups selected from the

1

to

3

groups listed below in lieu of a part of Fe:

1

Nb: 1.0% or less, V: 1.5% or less, Hf: 1% or less and B: 0.05% or less;

2

Mg: 0.05% or less, Ca: 0.05% or less, Y: 0.5% or less, La: 0.5% or less,Ce: 0.5% or less, Nd: 0.5% or less and Sc: 0.5% or less;

3

Ta: 8% or less, Re: 8% or less, Ir: 5% or less, Pd: 5% or less, Pt: 5%or less and Ag: 5% or less.
 5. A heat resistant pressure memberexcellent in creep resistance properties and structural stability in ahigh temperature range, which is made from the austenitic heat resistantalloy according to claim
 1. 6. A heat resistant pressure memberexcellent in creep resistance properties and structural stability in ahigh temperature range, which is made from the austenitic heat resistantalloy according to claim
 2. 7. A heat resistant pressure memberexcellent in creep resistance properties and structural stability in ahigh temperature range, which is made from the austenitic heat resistantalloy according to claim
 3. 8. A heat resistant pressure memberexcellent in creep resistance properties and structural stability in ahigh temperature range, which is made from the austenitic heat resistantalloy according to claim
 4. 9. A method for manufacturing the heatresistant pressure member excellent in creep resistance and structuralstability in a high temperature range, wherein the austenitic heatresistant alloy according to claim 1 is treated in sequence by thefollowing steps (i), (ii) and (iii): step (i): heating to 1050 to 1250°C. at least once before final hot or cold working; step (ii): carryingout a final hot or cold plastic working such that the reduction of areais 10% or more; step (iii): carrying out a final heat treatment in whichcooling is performed after heating and holding at a temperature in therange of 1100 to 1250° C.
 10. A method for manufacturing the heatresistant pressure member excellent in creep resistance and structuralstability in a high temperature range, wherein the austenitic heatresistant alloy according to claim 2 is treated in sequence by thefollowing steps (i), (ii) and (iii): step (i): heating to 1050 to 1250°C. at least once before final hot or cold working; step (ii): carryingout a final hot or cold plastic working such that the reduction of areais 10% or more; step (iii): carrying out a final heat treatment in whichcooling is performed after heating and holding at a temperature in therange of 1100 to 1250° C.
 11. A method for manufacturing the heatresistant pressure member excellent in creep resistance and structuralstability in a high temperature range, wherein the austenitic heatresistant alloy according to claim 3 is treated in sequence by thefollowing steps (i), (ii) and (iii): step (i): heating to 1050 to 1250°C. at least once before final hot or cold working; step (ii): carryingout a final hot or cold plastic working such that the reduction of areais 10% or more; step (iii): carrying out a final heat treatment in whichcooling is performed after heating and holding at a temperature in therange of 1100 to 1250° C.
 12. A method for manufacturing the heatresistant pressure member excellent in creep resistance and structuralstability in a high temperature range, wherein the austenitic heatresistant alloy according to claim 4 is treated in sequence by thefollowing steps (i), (ii) and (iii): step (i): heating to 1050 to 1250°C. at least once before final hot or cold working; step (ii): carryingout a final hot or cold plastic working such that the reduction of areais 10% or more; step (iii): carrying out a final heat treatment in whichcooling is performed after heating and holding at a temperature in therange of 1100 to 1250° C.